1. Field of the Invention
This invention relates to an electrophotographic apparatus such as a copying machine and a printer, and a charging apparatus used therein. More particularly, it relates to an electrophotographic apparatus in which a charging member is brought into contact with a photosensitive member to electrostatically charge the photosensitive member, and a charging apparatus.
2. Related Background Art
In charging apparatus used in electrophotography, corona charging assemblies have been used. In recent years, in place of them, contact charging assemblies are being put into practical use. The latter is intended for decreasing ozone and decreasing power consumption. In particular, roller charging systems employing a conductive roller as a charging member are preferably used in view of stability in charge.
In the roller charging, a conductive elastic roller is brought into pressure contact with an object member (e.g., photosensitive member) and a voltage is applied thereto to electrostatically charge the object member.
In the conventional contact charging, the object member is charged by the release of charges (i.e., discharging) from a charging member to the object member, and hence the charging takes place upon application of a voltage having a magnitude greater than a certain threshold voltage. For example, in an instance where a charging roller is brought into pressure contact with an OPC photosensitive member (a photosensitive member making use of an organic photoconductive material) of 25 .mu.m in layer thickness, the surface potential of the photosensitive member begins to increase upon application of a voltage of about 640 V or higher, and at voltages higher than that the surface potential of the photosensitive member linearly increases by gradient 1 with respect to the applied voltage. Hereinafter, this threshold voltage is defined as charge starting voltage Vth.
More specifically, in order to obtain a required surface potential Vd of the photosensitive member, it is necessary to apply to the charging roller a DC voltage of Vd+Vth. The method in which only a DC voltage is applied to a contact charging member to electrostatically charge the photosensitive member by discharging is called DC charging.
In the DC charging, however, it has been difficult to keep the surface potential of the photosensitive member at the desired value because the resistance value of the contact charging member may vary depending on environmental variations and also because the Vth may vary with changes in layer thickness due to the surface scrape of the photosensitive member with its use.
Accordingly, as a proposal to achieve more uniform charging, Japanese Patent Application Laid-open No. 63-149669 discloses an AC charging system in which a voltage formed by superposing on a DC voltage corresponding to the desired Vd an AC voltage having a peak-to-peak voltage of 2.times.Vth or higher is applied to the contact charging member. This system aims at an effect of leveling the potential by AC voltage, where the potential of the charging member is converged into the Vd that is the center of the peak of the AC voltage and can be hardly affected by external factors such as environment.
However, even in such a contact charging apparatus, its essential charging mechanism utilizes the phenomenon of discharging from the charging member to the photosensitive member. Hence, as previously stated the voltage required for the charging has a value greater than the surface potential of the photosensitive member and ozone is also generated in a very small quantity. Also, when the AC charging is effected in order to achieve the uniform charging, the ozone may more increase in quantity, the electric field of the AC voltage causes vibration or noise of the charging member and photosensitive member, or the surface of the photosensitive member may seriously deteriorate, bringing about additional problems.
Under such circumstances, it is desired to charge the photosensitive member in the manner that charges are directly injected into it without relying on the phenomenon of discharging, and some proposals are made on such direct injection of charges, none of which, however, have not yet put into practical use. Japanese Patent Application Laid-open No. 6-3921 proposes, as a more effective charge injection method, a method in which a charge injection layer is provided on the surface of a photosensitive member and charges are directly injected into that layer by means of a contact charging member (which is called injection charging).
In the injection charging, it is effective to use as the charging member a magnetic brush roller which can be brought into contact with the photosensitive member at a greater nip between them, and which can be brought into uniform contact with the surface of the photosensitive member and can be free from microscopic incomplete charging. This is to use a charging member having the form of a magnetic brush formed using a magnet roll to magnetically confine ferrite particles or medium-resistance charged magnetic particles obtained by dispersing magnetic fine particles in a resin.
The charge injection layer serving as a surface layer of the photosensitive member may be a layer formed by dispersing conductive fine particles in an insulating and light-transmitting binder. Such a layer is preferably used. The charging magnetic brush to which a voltage is applied comes in touch with this charge injection layer, whereupon the conductive fine particles come to exist as if they are numberless independent floating electrodes with respect to the conductive support of the photosensitive member, and can be expected to have such an action that they charge the capacitor formed by these floating electrodes.
Thus, the voltage applied to the contact charging member and the surface potential of the photosensitive member are converged into values substantially equal to each other, so that a low-voltage charging method can be accomplished.
However, in the above conventional case, where conventional magnetic resin particles are used as the charging magnetic particles, the magnetic resin particles having broken during use may become buried in the photosensitive member to tend to block exposure or affect charging performance.
Accordingly, it has been attempted to make up the magnetic resin particles using a resin with a high hardness so that the magnetic resin particles can have a higher strength. However, since the conventional magnetic resin particles are produced by kneading and pulverization, the surfaces of the magnetic resin particles may be macroscopically irregular to scratch the surface of the photosensitive member.
Such macroscopically irregular surfaces of the magnetic resin particles may also make the particles have a poor fluidity to make it difficult for the magnetic resin particles to smoothly come in touch with the photosensitive member to inject charges thereinto.
Moreover, in the injection charging, the charging member must microscopically well come in touch with the photosensitive member before the charges can be well injected. However, for the magnetic resin particles produced by pulverization and having macroscopically irregular surfaces, it has been difficult to well come in touch with the surface of the photosensitive member, resulting in an insufficient charging uniformity.
Meanwhile, in the case when charging magnetic particles comprised of only ferrite or magnetite which is a metal oxide are used, they may barely break when used. However, it is very difficult to produce charging magnetic particles having uniform small particle diameters. Hence, in an attempt to achieve a good charging uniformity, a fairly high production cost may result.
In addition, in the case of charging, which is different from developing, almost no toner is present between the magnetic particles and the photosensitive member, and hence there is the problem that the magnetic particles may scratch or scrape the photosensitive member.